20.1. Turn Off Reverse DNS Lookups

Reverse DNS maps IP numbers to machine names. The web server uses reverse DNS
for two things: writing a machine name rather than simply an IP address in the
web servers logs, and giving CGI programs easy access to the client machine
name. The CGI program can access the client machine name via the REMOTE_HOST
environment variable if reverse DNS is on.

The problems with reverse DNS are that it can take a long time and is generally
implemented as a blocking system call. This means that the current process or
thread can do no further work until the call either returns with an answer or
times out. While the time required generally isn't a problem for a single HTTP
request, it rapidly becomes the server bottleneck if there are more than a few
users making requests.